1. Field of the Invention
This invention relates to a ferroelectric liquid crystal display, and more particularly to a ferroelectric liquid crystal display and a driving method that is capable of preventing deterioration of light efficiency caused by a low voltage holding ratio.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) controls light in accordance with a liquid crystal alignment state to thereby display a desired picture on a screen. A liquid crystal used for such a LCD is in a neutral phase between a liquid state and a solid state, thereby having both a fluidity and an elasticity. In a thermodynamic phase transition process of the liquid crystal, for example, a liquid crystal having a smectic C phase is rotated along a smectic layer, taking a layer structure having the same electrical and magnetic property. The smectic C phase liquid crystal is rotated along an outer line of a virtual cone.
The smectic C phase liquid crystal has a characteristic of making a spontaneous polarization irrespectively of an external electric field. This type of liquid crystal is usually referred to as ferroelectric liquid crystal (FLC). The FLC has been actively studied because it has a fast response speed as a result of its spontaneous polarization characteristic. Accordingly, it has an ability to realize a wide viewing angle without a special electrode structure and a compensating film. In addition, the FLC includes a deformed helix FLC mode, a surface stabilized FLC mode, an anti-FLC mode, a V-type FLC mode and a half V-type FLC mode, etc. The V-type FLC mode and the half V-type FLC mode modes will be described.
FIG. 1 shows an alignment state of a liquid crystal cell in the V-type FLC mode.
Referring to FIG. 1, the liquid crystal cell, in the V-type FLC mode, includes an upper substrate 1 on which a common electrode 3 and an alignment film 5 are disposed, a lower substrate 11 on which a TFT array 9 including a pixel electrode and an alignment film 7, and a liquid crystal 13 injected between the upper and lower substrates 1 and 11. The alignment films 5 and 7 are aligned into a desired state by a rubbing method. The injected liquid crystal 13 forms a smectic layer, taking a layer structure, and is arranged into a phase having a desired slope with respect to a plane perpendicular to the smectic layer. The liquid crystal 13 has a desired inclination angle with respect to an aligned direction of the alignment film and is aligned such that the adjacent smectic layers have opposite polarities with respect to each other.
A transmissivity according to a voltage of the V-type FLC mode liquid crystal cell is shown in FIG. 2. The liquid crystal 13 within the V-type FLC mode liquid crystal cell responds to positive and negative voltages applied thereto. Since the transmissivity is suddenly changed according to an application of the positive and negative voltages, a transmissivity curve according to a voltage has a V shape. As shown in FIG. 2, transmissivity is increased as a positive voltage increases, whereas transmissivity is decreased as a negative voltage increases.
FIG. 3 shows an alignment state of a liquid crystal cell in the half V-type FLC mode.
In FIG. 3, a liquid crystal 15 within the half V-type FLC mode liquid crystal cell injected between the upper substrate 1 and the lower substrate 11 forms a smectic layer taking a layer structure. The liquid crystal 15 is aligned at a desired inclination angle with respect to an alignment treatment direction of the alignment films 5 and 7 such that the adjacent smectic layers have a different polarity, unlike the liquid crystal 13 in the V-type FLC mode. A half V-type mode liquid crystal can be implemented by applying a positive or negative electric field in advance while at the same time lowering its temperature into a temperature having a smectic phase. The half V-type FLC mode liquid crystal 15 formed in this manner responds to only one of the applied positive and negative voltages. Thus, as seen from FIG. 4, a transmittance curve according to a voltage of a liquid crystal cell in the half V-type FLC mode has a half V shape. A T-V characteristic in FIG. 4 represents the situation when a negative voltage is used to make an initial uniform alignment. In this case, transmissivity is almost not increased upon application of a negative voltage, whereas it is increased in accordance with an increase in a positive voltage. The opposite is true when a positive voltage is used to make an initial uniform alignment, that is, transmissivity is increased in accordance with an increase in a negative voltage.
A thermodynamic phase transition process of the half V-type FLC mode liquid crystal 15 is as follows:                Isotropic nematic (N*) phase→smectic C* (Sm C*) phase crystal        
As the temperature gradually decreases, the phase transition process of a liquid crystal goes from left to right as shown in the above thermodynamic phase transition.
For example, the liquid crystal 15 is aligned in parallel to a rubbing direction, when its temperature is slowly lowered to reach a temperature having a nematic phase after the liquid crystal 15 is injected into the liquid crystal cell at a temperature having an isotropic phase. If an electric field is applied to the interior of the cell while the temperature is slowly lowered, the liquid crystal 15 is phase-changed into a smectic phase. The direction of a spontaneous polarization of the liquid crystal 15 generated at this time is arranged in such a manner to be consistent with that of an electric field formed at the interior of the cell. As a result, when the liquid crystal 15 within the liquid crystal cell is subjected to a parallel alignment treatment, it takes a molecule arrangement in the spontaneous polarization direction, which is consistent with the direction of an electric field applied in said phase transition process of two possible molecule arrangement directions, thereby having a uniform alignment state.
Molecular arrangement in the spontaneous polarization direction will be described in detail with reference to FIG. 5 and FIG. 6 below. First, as seen from FIG. 5, if a negative electric field E(−) is applied to the alignment of the liquid crystal 15, then a spontaneous polarization direction of the liquid crystal 15 that is identical to the electric field direction is made, thus providing a uniform alignment. In the liquid crystal cell, as shown in FIG. 6, a liquid crystal arrangement is changed upon application of a positive electric field E(+), while a liquid crystal arrangement is not changed upon application of a negative electric field E(−). In order to utilize a response characteristic to an electric field of the liquid crystal 15, polarizers perpendicular to each other are arranged at the upper and lower portions of the liquid crystal cell. At this time, a transmission axis of one polarizer is arranged to be consistent with an initial liquid crystal alignment direction. In the liquid crystal cell having the above-mentioned arrangement, a transmission curve according to a voltage application has a half V shape as shown in FIG. 4 experimentally.
With respect to a negative electric field E(−), a liquid crystal arrangement is not changed to shut out a light. Otherwise, with respect to a positive electric field E(+), a liquid crystal arrangement is changed to transmit light. In this case, as a positive electric field E(+) increases, a transmittance also is increased.
Referring to FIG. 7, the ferroelectric LCD includes an upper substrate 102 on which a color filter array 104, a common electrode 106 and an alignment film 107 that has undergone an alignment treatment are sequentially disposed. A lower substrate 114, on which a pixel electrode 112 including a TFT array, and an alignment film 110 that has undergone an alignment treatment are sequentially disposed. Spacers (not shown) are provided between the upper substrate 102 and the lower substrate 114. Ferroelectric liquid crystals 108 are injected into an inner space between the upper and lower substrate 102 and 114 defined by the spacers. Polarizers 100 and 120 are attached at the outsides of the upper and lower substrates 102 and 114. A backlight 116 for irradiating a light and a backlight driver (not shown) for controlling a turn-on of the backlight 116 are provided.
The backlight driver applies an electrical signal to the backlight 116 to generate light. The backlight 116 creates a white light in response to the electrical signal from the backlight driver. The light generated from the backlight 116 is converted into a surface light source to be applied uniformly incident to the liquid crystal display panel. The white light from the backlight 116 is transmitted or blocked depending on an alignment state of the ferroelectric liquid crystals 108. For example, a voltage is applied to a certain pixel to generate a voltage difference between a pixel electrode 112 and a common electrode 106. Accordingly, a rotation angle of the liquid crystal molecules is changed and a transmittance is controlled in accordance with the changed rotation angle of the liquid crystal molecules, thereby implementing various black and white gray scales.
The light generated from such a backlight 116 transits the red, green and blue color filters 104 on the upper substrate 102 to have saturation and brightness. As illustrated in FIG. 8, one pixel has 3 sub-pixels for realizing a picture. The each sub-pixel corresponds to the pixel cell to sequentially form the red, green, blue color filter 104. The color filter 104 selectively transmits the red, green and blue wavelength corresponding to the specific wavelength of the white light to realize the colorful picture. The black matrix 118 is built between each of color filters 104 for each color not to interfere.
In the meantime, FIG. 9 shows the characteristics of the voltage holding ratio (VHR) of a ferroelectric liquid crystal. VHR refers to the ratio of holding the voltage charge in a liquid crystal cell after a voltage is applied to the liquid crystal cell. In other words, because the driving voltage is not applied to the liquid crystal cell during the non-selected period of LCD driving, the liquid crystal cell holds a floating state. The electric charge that is charged in the liquid crystal cell during the selected period upon the application of the driving voltage, is discharged to the outside of the liquid crystal cell during the non-selected period. VHR refers to the degree of the liquid crystal cell sustaining the voltage charge in the floating state. VHR characteristics of the ferroelectric liquid crystal is described as follows.
The ferroelectric liquid crystal has the characteristics of the spontaneous polarization, i.e., it has a polarity when a driving voltage is not applied. Thus, it rapidly restores the elasticity after the driving voltage has been applied. The ferroelectric liquid crystal is rotated to the position where the light is transmitted by the driving voltage initially supplied. However, the voltage charged to the ferroelectric liquid crystal drops below 50% of the voltage charge initially supplied in accordance with the characteristics of the depolarization of the liquid crystal. The voltage decreased is sustained for one frame period. The ferroelectric liquid crystal is rotated to the position where light is not transmitted by such a voltage decrease. Consequently, the time sustaining the position of the liquid crystal molecule transmitting the light, becomes less for the brightness to drop.
As a method for improving the VHR characteristics of the ferroelectric liquid crystal, the storage capacitance in the TFT design has been increased.
FIG. 10 shows the relationship of the VHR and the aperture ratio according to the storage capacitance (Cst).
Referring to FIG. 10, the VHR increases but the aperture ratio decreases when the size of the storage capacitance increases. In other words, when the size of the storage capacitor is increased to increase the value of the storage capacitance, the size of storage capacitor affects the pixel area to which the light is transmitted to thereby reduce the aperture ratio.
Accordingly, there is a limitation in the indefinite increase of the storage capacitance size to improve the VHR characteristics.
Also, decreasing the size of spontaneous polarization of the ferroelectric liquid crystal has been used as another way to improve the VHR characteristics of the ferroelectric liquid crystal.
FIG. 11 shows the relationship according to the spontaneous polarization of the ferroelectric liquid crystal.
Referring to FIG. 11, the VHR decreases when the spontaneous polarization increases. In other words, the VHR characteristics can be improved by decreasing the spontaneous polarization of the ferroelectric liquid crystal. However, the response time of the ferroelectric liquid crystal described in the Equation 1 should be considered when changing the size of the spontaneous polarization of the ferroelectric liquid crystal for the improvement of the characteristics of the VHR.τ=γ/(P*E)  [Equation 1]
In Equation 1, τ is the response time of the liquid crystal, γ is the rotational viscosity of the liquid crystal, P is the spontaneous polarization of the liquid crystal, E is the electric field.
As shown in the Equation 1, the response time of the ferroelectric liquid crystal has an inverse proportional relationship with the size of the spontaneous polarization of the ferroelectric liquid crystal. In other words, the response time of the ferroelectric liquid crystal decreases when the degree of the spontaneous polarization increases in the ferroelectric liquid crystal. An increase in VHR results in an increase in the leakage of the voltage charged to the liquid crystal. Whereas, the VHR decreases when the degree of the spontaneous polarization of the ferroelectric liquid crystal increases, and the response time of the ferroelectric liquid crystal increases. Therefore, when the size of the spontaneous polarization of the ferroelectric liquid crystal decreases the response time of the ferroelectric liquid crystal should also be considered.